E6581741
8. Modbus TCP server
8.1. Modbus TCP frames
Modbus TCP frames consist of a header and a Modbus request.
Header format:
Byte | Description |
|
| Comments |
0 | Transaction |
| high order |
|
1 | identifier |
| low order |
|
2 | Protocol |
| high order | This identifier always equals 0. |
3 | identifier |
| low order | |
|
| |||
4 | Length of data |
| high order | Number of bytes in the Modbus request +1. The frame length is |
5 |
| low order | always less than 256 bytes, the value of the significant byte | |
|
|
|
| therefore equals 0. |
6 | Destination identifier (Unit ID) |
| ||
7 | Modbus request | function |
| |
| code |
|
| |
The frame header returned by the
8.2. Drive Modbus servers
The destination identifier (Unit ID) is used to access drive Modbus TCP servers:
Unit ID | Modbus TCP server | Accessible parameters |
Variable speed drive | See the | |
251 | Option board |
|
255 | IO Scanner | See the “IO Scanner” section. |
8.3. List of Modbus functions supported
Function code | Modbus name | Description | Size of data | |
03 |
| Read Holding Registers | Read N output words | 63 words max. |
06 |
| Write Single Register | Write one output word | - |
16 | (0x10) | Write Multiple Registers | Write N output words | 63 words max. |
23 | (0x17) | Read/Write Multiple Registers | Read/write N words | 20/20 words max. |
|
|
| (IO Scanning) |
|
43 | (0x2B) | Read Device Identification | Identification | - |
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